Boost and linear led control

ABSTRACT

A light emitting diode (LED) lighting system includes a switching power converter having an input for coupling to an alternating current (AC) power source, an output, and a switch. The LED lighting system also includes an LED lighting subsystem coupled to receive power from the output of the switching power converter. The LED lighting subsystem includes a current source for one or more LEDs, and the current source has a control node and a sense node. The LED lighting system additionally includes a switch state controller coupled to the switching power converter and coupled to the LED lighting subsystem. The switch state controller controls switching of the switch and varies a control current provided to the control node of the current source based on at least a parameter sensed from the sense node.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit under 35 U.S.C. §119(e) and 37C.F.R. §1.78 of U.S. Provisional Application No. 61/410,259, filed Nov.4, 2010, and entitled, “Boost and Linear LED Control,” which isincorporated herein by reference.

The following additional U.S. patent applications are also incorporatedherein by reference:

1. U.S. patent application Ser. No. 13/194,531, filed Jul. 29, 2011, andentitled, “Powering High-Efficiency Lighting Devices From a Triac- BasedDimmer;” 2. U.S. patent application Ser. No. 13/194,699, filed Jul. 29,2011, and entitled, “Dimmer Compatibility with Reactive Loads;” 3. U.S.patent application Ser. No. 13/217,174, filed Aug. 24, 2011, andentitled, “Multi-Mode Dimmer Interfacing Including Attach StateControl;” 4. U.S. patent application Ser. No. 61/376,504, filed Aug. 24,2010, and entitled, “Multi-Mode Dimmer Interface for Lighting BoostController;” 5. U.S. patent application Ser. No. 61/410,168, filed Nov.4, 2010, and entitled, “Dimmer Dump Circuits and Controls;” 6. U.S.patent application Ser. No. 61/410,269, filed Nov. 4, 2010, andentitled, “Dimmer Interface for Low Power Lamps;” 7. U.S. patentapplication Ser. No. 61/414,291, filed Nov. 16, 2010, and entitled,“Trailing Edge Boost Compatibility Circuit; 8 U.S. patent applicationSer. No. 61/437,355, filed Jan. 28, 2011, and entitled, “PoweringHigh-Efficiency Lighting Devices From a Triac- Based Dimmer;” and 9.U.S. patent application Ser. No. 61/503,369, filed Jun. 30, 2011, andentitled, “Dimmer Compatible LED Lamps with Secondary Side CurrentControl.” 10. U.S. patent application Ser. No. 13/287,257, filed Nov. 2,2011, and entitled, “Duty Factor Probing of a TRIAC-Based Dimmer; and”11. U.S. patent application Ser. No. _(——) _(——) _(——) (MOI 1882), filedon even date herewith, and entitled, “Digital Resynthesis of InputSignal for Dimmer Compatibility.” 12. U.S. patent application Ser. No.13/289,816, filed Nov. 4, 2011, and entitled, “Boost and Linear LedControl.”

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to the field of electronics andlighting, and more specifically, to a system and method for controllingand/or providing power to current regulated light sources, such as lightemitting diode light sources.

2. Description of the Related Art

Commercially practical incandescent light bulbs have been available forover 100 years. Recently, other light sources have begun to show promiseas commercially viable alternatives to the incandescent light bulb. Forexample, light emitting diodes (LEDs) are becoming particularlyattractive as light sources in part because of long life, energy savingsthrough high efficiency light output, and environmental incentives suchas the reduction of use of mercury.

LEDs are semiconductor devices and are driven by direct current. Thebrightness of an LED varies in direct proportion to the current flowingthrough the LED. Thus, increasing current supplied to an LED increasesthe brightness of the LED, and decreasing current supplied to the LEDdims the LED.

FIG. 1 illustrates a representative prior art LED lighting system 100,which includes a switching power converter 102. Voltage source 101supplies an alternating current (AC) input voltage V_(in)(t) to a fullbridge rectifier 103. Voltage source 101 can be, for example, a publicutility, and the AC voltage V_(in)(t) is, for example, a 60 Hz/110 Vline voltage in the United States of America or a 50 Hz/230 V linevoltage in Europe. Full bridge rectifier 103 rectifies the input voltageV_(in)(t) and supplies a rectified, time-varying, line input voltageV_(x)(t) to switching power converter 102.

Switching power converter 102 includes a switch 108 that operates inresponse to a control signal C_(S) to regulate the transfer of energyfrom the rectified, time-varying input voltage V_(x)(t), throughinductor 110 to capacitor 106. Switching power converter 102additionally includes a diode 111 that prevents reverse current flowfrom capacitor 106 into inductor 110.

Energy transferred through inductor 110 is stored by capacitor 106.Capacitor 106 has sufficient capacitance to maintain an approximatelyconstant voltage V_(C) while providing current to a load, such as LEDlighting subsystem 112. In at least some implementations, switchingpower converter 102 is a boost-type converter in which voltage V_(C) isgreater than the peak of input voltage V_(x)(t).

In operation, input current i_(in) varies over time, with a peak inputcurrent proportionate to the “on-time” of switch 108 and with the energytransferred to capacitor 106 proportionate to the “on-time” squared.Thus, this energy transfer process is one example of a nonlinearprocess. In some implementations, switch 108 is an n-channel fieldeffect transistor (FET), and control signal C_(S) is a pulse widthmodulated (PWM) control signal that causes switch 108 to conduct whenthe pulse width of C_(S) is high. Thus, in such implementations, the“on-time” of switch 108 is determined by the pulse width of controlsignal C_(S), and the energy transferred from V_(x)(t) to capacitor 106is proportionate to a square of the pulse width of control signal C_(S).

Power control system 100 also includes a switch state controller 114that generates control signal C_(S) with a goal of causing switchingpower converter 102 to transfer a desired amount of energy to capacitor106, and thus, to LED lighting subsystem 112. The desired amount ofenergy depends upon the voltage and current requirements of LED lightingsubsystem 112. To provide power factor correction close to one, switchstate controller 114 generally seeks to control input current i_(in) sothat input current i_(in) tracks input voltage V_(x)(t) while holdingcapacitor voltage V_(C) substantially constant.

In practical systems, however, link voltage V_(L) is subject to ripple,which can cause visible flicker in the LEDs of the LED lightingsubsystem 112. Such flicker will become even more severe if LED lightingsystem 100 includes a phase-cut dimmer to dim the LEDs of LED lightingsubsystem 112. The present application recognizes that it would bedesirable to efficiently operate an LED lighting subsystem off of linevoltage with reduced link voltage-induced ripple and flicker.

SUMMARY OF THE INVENTION

In at least one embodiment, a light emitting diode (LED) lighting systemincludes a switching power converter having an input for coupling to analternating current (AC) power source, an output, and a switch. The LEDlighting system also includes an LED lighting subsystem coupled toreceive power from the output of the switching power converter. The LEDlighting subsystem includes a current source for one or more LEDs, andthe current source has a control node and a sense node. The LED lightingsystem additionally includes a switch state controller coupled to theswitching power converter and coupled to the LED lighting subsystem. Theswitch state controller controls switching of the switch and varies acontrol current provided to the control node of the current source basedon at least a parameter sensed from the sense node.

In another embodiment, a LED lighting subsystem includes one or moreLEDs and a current source for the one or more LEDs. The current sourceincludes a transistor having a control input coupled to a constantvoltage source, a first leg coupled to the one or more LEDs and a secondleg. The current source additionally includes a current mirror coupledto the second leg of the transistor. The current mirror has a controlnode, a sense node indicative of an operating point of the LED lightingsubsystem, and a transistor size ratio that determines an LED currentprovided to the one or more LEDs based on a control current received atthe control node.

In another embodiment, a lighting system controller for a LED lightingsystem illuminating one or more LEDs from AC line voltage receives alink voltage and a feedback signal indicative of an operating point ofthe LED lighting system. Responsive thereto, the lighting systemcontroller varies a control current provided to a current source thatgoverns current through the one or more LEDs such that ripple in the oneor more LEDs is no more than a predetermined amount.

In another embodiment, a method for operating a LED lighting systemincludes providing a link voltage to a LED lighting subsystem through aswitching power converter coupled to an alternating current (AC) powersource. The LED lighting subsystem includes one or more LEDs, a sensenode, and a current source for one or more LEDs, where the currentsource has a control node. The current source generates an LED currentfor the one or more LEDs based on a control current received at thecontrol node of the current source. A control signal that controlsswitching of a switch of the switching power converter is generated thatvaries the control current provided to the control node of the currentsource based on at least a parameter sensed from the sense node.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a representative prior art LED lighting system.

FIG. 2 depicts a representative LED lighting system.

FIG. 3 is a timing diagram illustrating optimization of the operatingpoint of a LED lighting system through link voltage regulation.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

With reference again to the figures and with particular reference toFIG. 2, there is illustrated a high level block diagram of arepresentative LED lighting system 200 having improved link voltageregulation for optimized operation. LED lighting system 200 is designedto operate off the line and accordingly includes an alternating current(AC) input 202 that may be coupled to a voltage source 201 that suppliesAC input voltage V_(in)(t). Voltage source 201 can be, for example, apublic utility, in which case, voltage V_(in)(t) can be, for example, a60 Hz/110 V line voltage in the United States of America or a 50 Hz/230V line voltage in Europe.

LED lighting system 200 optionally but preferably includes a phase-cutdimmer 204 that enables selection of an average AC input power deliveredto the remainder of LED lighting system 200. Although phase-cut dimmer204 can take various different embodiments, in at least one preferredembodiment, phase-cut dimmer 204 is triac-based, as described in U.S.patent application Ser. No. 13/194,531, referenced above.

LED lighting system further includes a switching power converter 210coupled to receive AC power from AC power input 202 and phase-cut dimmer204 (if present) and to output a link voltage V_(L). Switching powerconverter 210 includes a full bridge rectifier 212 that rectifies theinput voltage V_(in)(t) (as modified by phase-cut dimmer 204, ifpresent) to supply a rectified, time-varying, line input voltageV_(x)(t). An exemplary input waveform of V_(x)(t) as modified by aphase-cut dimmer 204 implementing leading edge (forward phase) dimmingis illustrated at reference numeral 300 of FIG. 3. In other embodiments,phase-cut dimmer 204 can implement trailing edge (reverse phase)dimming, which truncates the trailing edge rather than the leading edgeof the input AC voltage waveform.

Similar to switching power converter 102 of FIG. 1, switching powerconverter 210 includes a switch 216 that turns “on” (conducts) and turns“off” (nonconductive) in response to a control signal C_(S1) in order toregulate the transfer of energy from the rectified, time-varying inputvoltage V_(x)(t), through an inductor 214 to a capacitor 218. Whenswitch 216 is “on,” inductor 214 energizes with the current i_(in) fromfull-bridge diode rectifier 212. When switch 216 is “off,” inductor 214drives current i_(in) to charge capacitor 218 through diode 217, whichprevents reverse current flow from capacitor 218 into inductor 214. Anexemplary waveform of i_(in) is illustrated at reference numeral 302 ofFIG. 3. Capacitor 218 preferably has sufficient capacitance to maintainan approximately constant capacitor voltage V_(C1) (for a given inputvoltage V_(x)(t)) while providing current to a load. In at least someimplementations, switching power converter 210 is a boost-type converterin which capacitor voltage V_(C1) is greater than the peak of inputvoltage V_(x)(t).

The input current i_(in) flowing through inductor 214 is proportionateto the “on-time” of switch 216, and the energy transferred by switchingpower converter 210 to capacitor 218 is proportionate to the switch“on-time” squared. In at least one representative embodiment, controlsignal C_(S1) is a pulse-width modulated (PWM) signal, and switch 216 isan n-channel field effect transistor (FET) that conducts (is “on”) whenC_(S1) is high and does not conduct (is “off”) when C_(S1) is low. Thus,in this embodiment the “on-time” of switch 216 is determined by thepulse width of control signal C_(S1), and the energy transferred tocapacitor 218 is proportionate to a square of the pulse width of controlsignal C_(S1).

Still referring to FIG. 2, LED lighting system 200 includes an LEDsubsystem 230 coupled to the output of switching power converter 210.LED subsystem 230 includes one or more individual series-connected LEDsor one or more parallel-coupled strings of LED(s), generally depicted atreference numeral 232. The light output by LED(s) 232 is determined bythe LED current i_(LED) supplied to LED(s) 232.

LED(s) 232 are further coupled in series with a current source in LEDsubsystem 230. In the depicted representative embodiment, the currentsource includes an NPN power transistor Q1 cascaded at its emitter witha current mirror formed by transistors Q2 and Q3. Power transistor Q1 isunswitched during normal operation (i.e., after power has been suppliedto the LED lighting system 200 and any initial voltage or currenttransients have subsided), with its gate coupled via resistor 234 to afirst DC reference voltage, such as 5 V. The current mirror includes acontrol node 240 to which a control current i_(ctrl) is supplied, and asense node 242 at the collector of power transistor Q1 at which a sensevoltage V_(sense) is sensed. The current mirror determines LED currenti_(LED) based on the control current i_(ctrl) in accordance with theratio of sizes of transistors Q3 and Q2 as follows:

i _(LED)=_(ctrl)×size(Q3)/size(Q2)

An exemplary waveform of i_(LED) is illustrated at reference numeral 306of FIG. 3.

LED lighting system further includes a feedback circuit to enablecontrol of the operating point of LED lighting system 200. In thedepicted embodiment, the feedback circuit is implemented with acomparator 250 that compares the sense voltage V_(sense) present atsense node 242 of LED subsystem 230 with a second DC reference voltage,such as 4 V. The resulting feedback signal V_(comp) represents with itsamplitude and duration the present operating point of LED lightingsystem 200.

As further depicted in FIG. 2, LED lighting system 200 includes alighting system controller 260, which may be implemented, for example,in digital or analog circuitry or a combination thereof and may furtherbe implemented as an integrated circuit (IC) (which may optionallyadditionally include one or more other components of LED lighting system200). In addition to feedback signal V_(comp), lighting systemcontroller 260 is coupled to receive time-varying input voltage V_(x)(t)and link voltage V_(L). Based on these inputs, lighting systemcontroller 260 controls control current i_(ctrl) and the pulse width andperiod of control signal C_(S1). Specifically, lighting systemcontroller 260 determines and outputs the appropriate control currenti_(ctrl) to set LED current i_(LED) at an approximately constant level,where the approximately constant level corresponds to the dimming levelof phase-cut dimmer 204. The dimming level of the phase-cut dimmer 204is indicated by the duration of the “cut” in input voltage V_(x)(t)relative to the period of input voltage V_(x)(t). In addition, lightingsystem controller 260 controls control signal C_(S1) so that linkvoltage V_(L) satisfies the voltage requirements of LED subsystem 230and keeps sense node 242 at a voltage sufficiently high to keep thecurrent source comprising transistors Q1, Q2 and Q3 at least mostly “on”and to thus maintain the desired LED current i_(LED). For efficiency,lighting system controller 260 further preferably selects the targetV_(L) to be low enough so that LED current i_(LED) can just bemaintained with less than or equal to a predetermined amount of ripple(which may be zero). An exemplary waveform of V_(L) is illustrated atreference numeral 304 of FIG. 3.

Thus, lighting system controller 260 controls the nonlinear powertransfer process of switching power converter 210 so that capacitor 218is transferred a desired amount of energy that satisfies the voltage andcurrent requirements of LED subsystem 230. The duty cycle of controlsignal C_(S1) is set to maintain the desired capacitor voltage V_(C1)(e.g., between 200 and 400 V) and load voltage V_(L), and in at leastone embodiment, the duty cycle D of control signal C_(S1) equals[V_(L)/(V_(C1)+V_(L))]

Energy transfer increases during a period of time as the input voltageV_(x)(t) increases. To regulate the amount of energy transferred andmaintain a power factor from 0.7 to close to one, lighting systemcontroller 260 varies the period of control signal C_(S1) so that theinput current i_(in) tracks the changes in input voltage V_(x)(t) andholds the capacitor voltage V_(C1) substantially constant. For example,in one embodiment, switch 216 is controlled so that input current i_(in)is linearly proportional to input voltage V_(x)(t). Thus, as the inputvoltage V_(x)(t) increases, lighting system controller 260 increases theperiod T of control signal C_(S1), and as the input voltage V_(x)(t)decreases, lighting system controller 260 decreases the period T ofcontrol signal C_(S1). At the same time, the pulse width PW of controlsignal C_(S1) is adjusted to maintain a constant duty cycle D, and thus,hold capacitor voltage V_(C1) substantially constant. In at least oneembodiment, lighting system controller 260 updates control signal C_(S1)at a frequency much greater than the frequency of input voltageV_(x)(t). Because input voltage V_(x)(t) is generally a rectified linevoltage of a public utility, the frequency of input voltage V_(x)(t) isgenerally 50-60 Hz. The frequency 1/T of control signal C_(S1) is, forexample, between 25 kHz and 100 kHz. Frequencies at or above 25 kHzavoid audio frequencies and at or below 100 kHz avoid significantswitching inefficiencies while still maintaining good power factorcorrection (e.g., between 0.7 and 1) and an approximately constantcapacitor voltage V_(C1). Lighting system controller 260 may trade offpower factor for efficiency.

In some embodiments, LED lighting system 200 may receive excess inputenergy and therefore need to shed excess energy through its components.Techniques for discarding excess energy through the components of a LEDlighting system are described, for example, in U.S. Patent ApplicationSer. No. 61/410,168, referenced above. It should be noted that onecomponent that can be utilized to discard excess energy in accordancewith the disclosed techniques is power transistor Q1, which can discardexcess energy by making load voltage V_(L) higher than necessary.

As has been described, in one embodiment, light emitting diode (LED)lighting system includes a switching power converter having an input forcoupling to an alternating current (AC) power source, an output, and aswitch. The LED lighting system also includes an LED lighting subsystemcoupled to receive power from the output of the switching powerconverter. The LED lighting subsystem includes a current source for oneor more LEDs, and the current source has a control node and a sensenode. The LED lighting system additionally includes a switch statecontroller coupled to the switching power converter and coupled to theLED lighting subsystem. The switch state controller controls switchingof the switch and varies a control current provided to the control nodeof the current source based on at least a parameter sensed from thesense node.

While the present invention has been particularly shown as describedwith reference to one or more preferred embodiments, it will beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention.

What is claimed is:
 1. A light emitting diode (LED) lighting subsystem,comprising: one or more LEDs; a current source for the one or more LEDs,the current source including: a transistor having a control inputcoupled to a constant voltage source, a first leg coupled to the one ormore LEDs and a second leg; and a current mirror coupled to the secondleg of the transistor, the current mirror having a control node, a sensenode indicative of an operating point of the LED lighting subsystem, anda transistor size ratio that determines an LED current provided to theone or more LEDs based on a control current received at the controlnode.
 2. The LED lighting subsystem of claim 1, wherein the one or moreLEDs include multiple parallel coupled strings of LEDs.
 3. The LEDlighting subsystem of claim 1, wherein the transistor is a NPNtransistor.
 4. An apparatus comprising: the LED lighting subsystem ofclaim 1; a feedback circuit coupled to the sense node; and a lightingsystem controller that supplies the control current to the control nodeof the current mirror.
 5. The apparatus of claim 4, wherein the lightingsystem controller varies the control current provided to the controlnode based on an amount of dimming applied to the one or more LEDs by aphase-cut dimmer.
 6. The apparatus of claim 4, wherein the switch statecontroller varies the control current provided to the control node toconstrain current ripple in the one or more LEDs to no more than apredetermined amount.
 7. A lighting system controller for a lightemitting diode (LED) lighting system illuminating one or more LEDs fromAC line voltage, wherein the lighting system controller receives a linkvoltage and a feedback signal indicative of an operating point of theLED lighting system, and responsive thereto, varies a control currentprovided to a current source that governs current through the one ormore LEDs such that ripple in the one or more LEDs is no more than apredetermined amount.
 8. The lighting system controller of claim 7,wherein the lighting system controller further receives as an input asignal indicative of phase-cut dimming applied to the LED lightingsystem, and responsive thereto, varies the control current based on thesignal indicative of phase-cut dimming.
 9. The lighting systemcontroller of claim 8, wherein the signal indicative of phase-cutdimming is a rectified input voltage of the LED lighting system.
 10. Thelighting system controller of claim 7, wherein the lighting systemcontroller comprises an integrated circuit.